Do the fight on platforms rather than actual blocks. It'll render the cannon as only a melee threat.
Terraria Community Forums
Vinnoe
Snickerbobble
I usually like to take out the laser as it can be a big threat but it can be destroyed fairly quickly. And yeah, cannon won't even bother you, so then you'll only have two arms to worry about. If its passed half health before midnight, you can afford to attack the other arms to make the fight even easier.
Tord21
keep moving, use the Scourge of the Corrupter, Rainbow gun, or Vampire Knives if you have them, but definitely use the Terra Blade. Make sure you have your minions spawned in